Latex binder for nonwoven fibers and article made therewith

ABSTRACT

The invention relates to a latex that may be utilized as a binder for nonwoven applications to form a permeable sublayer of personal hygiene articles. The latex is prepared by a process including the steps of (1) polymerizing a monomer mixture comprising styrene, itaconic acid, surfactant and water soluble free radical initiator to form a seed; (2) sequentially adding equal increments of a monomer mixture of styrene, butadiene and acrylic acid to the seed under emulsion polymerization conditions to form a styrene-butadiene-acrylic acid copolymer; and then (3) neutralizing the styrene-butadiene-acrylic acid copolymer to a pH of about 4.5 to 7 to form the latex.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/921,241, entitled Latex Binder For NonwovenFibers And Article Made Therewith, filed Aug. 2, 2001, which is aDivisional of U.S. patent application Ser. No. 09/368,555, now U.S. Pat.No. 6,337,359, entitled Latex Binder For Nonwoven Fibers And ArticleMade Therewith, filed Aug. 5, 1999, the disclosures of which are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to a latex binder for treatingnonwoven fibers and a method of forming a permeable membrane includingthe latex binder. More particularly, the present invention relates to alatex binder for treating nonwoven fibers to make a durable ordisposable nonwoven fabric that is particularly suitable for use as apermeable sublayer of a personal hygiene article.

BACKGROUND OF THE INVENTION

[0003] A nonwoven fabric is a web or continuous sheet of fibers laiddown mechanically. The fibers may be deposited in a random manner ororiented in one direction. Most widely used fibers include cellulosicsand synthetics including polyamides, polyesters, polypropylene andpolyethylene. The spun fibers, which may be drawn, are laid downdirectly onto a belt by carding, airlaying or wet-laying. The sheet isthen bonded together with a latex binder and subsequently treated in anoven or a calendar to complete the bonding process.

[0004] Commonly used lattices for nonwoven fabrics are those preparedfrom polymers of butadiene-styrene, butadiene-acrylonitrile, vinylacetate, acrylic monomers such as methyl acrylate, ethyl acrylate,methyl methacrylate and the like. It will be appreciated that to beuseful as a nonwoven fabric in many applications, such as a permeablesublayer of a personal hygiene article, the latex binder should possessseveral desired physical properties. The desired physical propertiesinclude adequate tensile strength, a high modulus or stiffness undercertain conditions, and good textile qualities such as tenacity, handleand the like.

[0005] It is an object of the present invention to provide a nonwovenfabric having a high modulus. It is another object of the presentinvention to provide a nonwoven fabric having desired physicalproperties. Yet another object of the present invention is to provide anonwoven fabric of chemically bonded fibers that is simple andeconomical to manufacture.

SUMMARY OF THE INVENTION

[0006] Briefly, the present invention relates to a latex that isparticularly suitable for utilization as a binder for nonwovenapplications. The latex is prepared by a process including the steps of:

[0007] (1) polymerizing a monomer mixture comprising styrene, itaconicacid, surfactant and water soluble free radical initiator to form aseed;

[0008] (2) adding a monomer mixture of styrene, butadiene and chaintransfer agent and reacting the mixture to 40-95% conversion andpreferably 80-95% conversion;

[0009] (3) sequentially adding equal increments of a monomer mixture ofstyrene, butadiene, acrylic acid and optionally other functionalmonomers, in the presence of a chain transfer agent to (2) underemulsion polymerization conditions to more than 99% conversion; and then

[0010] (4) neutralizing the copolymer from (3) to a pH of about 4.5 to 7to form the latex.

[0011] The latex is particularly suitable to treat nonwoven fibers toform a permeable sublayer for use in both durable or disposablenonwovens. In a preferred embodiment, the permeable sublayer istypically interposed between a topsheet layer and a backsheet layertypically used to form a personal hygiene article.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The present invention relates to a latex suitable for utilizationas a binder to chemically bond fibers and form a nonwoven fabric. Thenonwoven fabric is particularly suitable for use as a permeable sublayerof a durable or disposable nonwoven. In a preferred embodiment, thepermaeable sublayer may be used as a personal hygiene article of a typewell known in the art. Examples of personal hygiene articles includediapers, feminine hygiene articles and the like.

[0013] The permeable sublayer is a web or mat comprised of randomlyarranged nonwoven fibers having an open structure and high loft. The webmay be formed by carding when the fibers are of such a character, byvirtue of their length and flexibility, as to be capable of carding.During carding the crimped fibers are placed on a moving support andthen treated with a latex binder as described herein.

[0014] The fibers may include natural textile fibers such as jute,sisal, ramie, hemp, and cotton as well as many artificial organictextile fibers or filaments including rayon, those of cellulose esterssuch as cellulose acetate, vinyl resin fibers such as those of polyvinylchloride, copolymers of vinyl chloride with vinyl acetate, vinylidenechloride or acrylonitrile, copolymers of acrylonitrile with vinylchloride, vinyl acetate, methacrylonitrile, vinyl pyridine, polymers andcopolymers of olefins such as ethylene and propylene, also condensationpolymers such as polyamides or nylon types, polyesters such as ethyleneglycol terephthalate polymers and the like.

[0015] The fibers may be of one composition or mixtures of fibers in agiven web. The preferred fibers include polyolefins, especiallypolyesters, poly(ethylene terephthalate), and acrylics, polyamides,polypropylene. The polyolefin fibers include polypropylene,polyethylene, polybutene and their copolymers. The polyester fibersinclude any long chain synthetic polymer composed of at least 85% byweight of an ester of a dihydric alcohol and terephthalic acid such aspolyethylene terephthalate, and, in addition liquid crystal polyesters,thermotropic polyesters and the like. The acrylic fibers include anyfiber forming substance containing a long chain synthetic polymercomposed of at least 85% by weight acrylonitrile units —CH₂ CH(CN)—. Itwill be appreciated that other types of fibers may also be employed inaccordance with the teachings of the present invention. For example,high modulus fibers more commonly known as graphite fibers made fromrayon, polyacrylonitrile or petroleum pitch may also be used.

[0016] The fibers may be of most any suitable size and randomly arrangedto most any suitable thickness depending upon the desired end use of thenonwoven fabric. The fibers are typically of a length of about 0.25 to 2inches and typically about 1.2-15 denier. The fibers may be laid in anoverlapping, intersecting random arrangement to a thickness of about0.25 inches or less to form a mat of fibers. The fibers may be arrangedby most any convenient known manner such as by wet laying, air-laying orcarding.

[0017] The fabric of the present invention is made by forming a mat ofrandomly arranged fibers. After the fibers are randomly arranged asdesired, a latex binder is applied to the fibers. The latex binder isapplied in an effective amount which will result in the fabric havingsufficient strength and cohesiveness for the intended end useapplication. As well known in the art, the latex binder may be appliedto the layer of randomly arranged fibers in a spaced, intermittentpattern of binder sites, or uniformly applied throughout the layer offibers. It will be further appreciated that the exact amount of thelatex binder employed depends, in part, upon factors such as the type offiber, weight of fibrous layer, nature of latex binder and the like. Forexample, end uses which require a stronger fabric may utilize morebinder. A typical content of latex binder applied on a fiber mat isabout 15-40 wt %. It is preferred that the minimum amount of latexbinder be applied to obtain the desired required physical properties ofthe nonwoven fabric such as tensile, hand and the like as well known inthe art.

[0018] The latex is then cured using methods well known in the art suchas by application of heat or radiation. The term “cured” refers to thelatex being cross-linked to increase the tensile strength properties.The curing of the treated fibers is affected at a temperature above theglass transition temperature of the binder.

[0019] To obtain a sufficient thickness for the end use intended aplurality of webs may also be superimposed upon one another to achieve asufficient thickness. The webs may be superimposed by alternating layersof carded webs disposed with their fiber orientation directions disposedat angles such as 60 degrees or 90 degrees with respect to interveninglayers.

[0020] The latex binder utilized in accordance with the presentinvention may be prepared by well-known conventional emulsionpolymerization techniques using one or more ethylenically unsaturatedmonomers and a polymeric surfactant as herein disclosed and additionalconventional additives such as free-radical initiators, optional chaintransfer agents, chelating agents and the like can be utilized as setforth in U.S. Pat. No. 5,166,259 to Schmeing and White.

[0021] In accordance with a preferred embodiment of the presentinvention, the latex is prepared by polymerizing a monomer mixturecomprising styrene, itaconic acid, surfactant and a water soluble freeradical initiator to form a seed.

[0022] A monomer mixture comprising styrene, butadiene and chaintransfer agent is then added to the seed and reacted to 40-95%conversion. In a preferred embodiment, the mixture is reacted to 80-95%conversion.

[0023] A monomer mixture including styrene, butadiene, acrylic acid andoptionally other functional monomers and chain transfer agent are addedincrementally in stages to the mixture to form a copolymer. Ifnecessary, additional surfactant, water and or persulfate initiators maybe added between increments. The acrylic acid and optional otherfunctional monomers are added to functionalize the polymer with acidand/or hydroxyl functional materials.

[0024] The optional functional monomers include hydroxyethylacrylate,acrylamide, methacrylic acid, and N-methylolacrylamide and mixturethereof. In preferred embodiment, the functional monomers are between0-10 wt % and preferably 0.1-5 wt %.

[0025] In a preferred embodiment, the monomer mixture includes about20-80 wt % styrene and 20-80 wt % butadiene of the total composition.The monomer mixture also includes at least about 2.0 wt % itaconic acidof the total composition, and acrylic acid in an amount of about 0.25-6wt %, preferably 2-4 wt %.

[0026] A surfactant is added to the monomer mixture in an amount ofabout 0.05-2.0 wt %. The surfactant may be most any suitable emulsifier,soap, or the like well known in the art and suitable at the pH of thelatex in accordance with the present invention. Examples of suitableemulsifiers and surfactants include alkyl sulfates, alkylsulfosuccinates, alkyl aryl sulfonates, alpha-olefin sulfonates, fattyor rosin acid salts, only or octyl phenol reaction products of ethyleneoxide and the like. Other surfactants that may be used include thoseidentified in Surface Active Agents, Schwartz and Berry, Vol. 1,Interscience Publishers, Inc., New York, 1958; Surface Activity, Moilet,Collie and Black, D. Van Nostrand Company, Inc., New York, 1961; OrganicChemistry, Feiser and Feiser, D.C. Heath and Company, Boston, 1944; andThe Merck Index, Seventh Edition, Merck & Co., Inc., Rahway, N.J., 1960,all of which are hereby incorporated by reference.

[0027] The copolymer is then neutralized to a pH of about 4.5 to 7.0 toform the latex. The pH of the latex is neutralized by addition of abase. Examples of a suitable base include potassium hydroxide, sodiumbicarbonate, ammonium hydroxide, sodium hydroxide and the like. Theamount of base added to the latex is adjusted to obtain the desired pHrange as is well known in the art.

[0028] Polymerization is typically carried out from about 120 degreesFahrenheit to 185 degrees Fahrenheit. Polymerization is generallyconducted for about 4 to 24 hours, however polymerization conditions mayvary as desired to provide different conversion levels of monomer tocopolymer. The final monomer mixture is allowed to react untilsubstantially constant solids at which time at least 99% of the monomershave been converted.

[0029] The invention will be further clarified by a consideration of thefollowing example, which is intended to be purely exemplary of theinvention.

EXAMPLE

[0030] Initially, a charge of deionized water, styrene and itaconicacid, and water soluble free radical initiator were added to a reactorhaving a volume of about 20 gallons and having a capacity to hold about140 lb. of latex. The reactor was then evacuated with a vacuum (about 20inches of mercury), purged with nitrogen and heated to a temperaturebetween 120-180 degrees Fahrenheit. This initial charge is allowed toreact to a desired conversion of between 50-99% of the total amount ofreactive monomers present. A charge comprising styrene, butadiene andchain transfer agent, preferably Sulfole 120, was then added to thereactor. The charge is reacted to a conversion level of between 50-95%conversion

[0031] Subsequent charges of styrene, Sulfole 120, acrylic acid andbutadiene were then charged to the reactor sequentially and reacted tothe desired conversion level The first increment was added approximately10-30 minutes after the reaction initiation takes place. Additionalincrements were then added to the reactor at staged intervals of about15 or 20 minutes or as dictated by % conversion The increments may beadded over most any suitable number of staged intervals depending uponthe amount of latex binder to be polymerized. For example, theincrements may be added from 6 stages up to 12 or more stages. After thelast increment was added to the reactor the reaction was monitored untilthe solids level of the latex in the reactor indicated an acceptableconversion level. Post reaction treatments include steam stripping withvacuum to remove excess un reacted monomers and nonreactable VOCs. Postreaction addition as of initiators may be added to further reduced theamount of unreacted monomer.

[0032] The resulting latex binder was then applied to polyester fibers.After applying the latex binder to the nonwoven fibers the latex binderwas oven dried at 212 degrees Fahrenheit to bond the fibers and thencured at 300 degrees Fahrenheit for 1 minute to form a dimensionallystable nonwoven fabric.

[0033] Representative physical properties of the styrene-butadiene latexbinder are shown in Table 1. As shown in Table 1, the fibers had ahigher hand value and higher dry tensile properties as compared topolyester fibers treated with GenFlo 3060. GenFlo 3060 is a styrenebutadiene rubber latex commercially available from OMNOVA Solutions Inc.TABLE 1 GenFlo 3060 Latex Latex content (%) 39.5 39.9 Yellow Index −7.6−7.7 Hand Value 44.3 68.8 Cross Direction, dry Tensile (grams) 323 503Elongation (%) 27 17 TEA 199 206 Cross Direction, wet Tensile (grams)162 141 Elongation (%) 39 4 TEA 79 10

[0034] All of the reported performance properties were determined afterconditioning the fibers for about 24 hours at TAPPI (TechnicalAssociation of the Pulp and Paper Industry) Standard Conditions ofapproximately 72 degrees Fahrenheit and about 50% relative humidity. Thetensile values, both dry and wet, were determined in accordance withASTM D 1117-80 entitled “Standard Methods of Testing Nonwoven Fabrics”published in the 1980 Annual Book of ASTM Standards. Following the ASTMstandard test method, dry tensile measurements were determined by using1 inch wide and 4 inch long strips of fabric pulled at a rate of 5inches per minute at an initial jaw separation of about 3 inches on anInstron. The wet tensile measurements were determined in substantiallythe same manner as the dry tensile measurements except the fabric stripwas soaked in water solution for about 30 seconds prior to testing onthe Instron. The Hand Values are a quantitative measure of the fabric aswell known in the textile industry. The Hand Values reported are anaverage value of the readings determined on a Thwing AlbertHandle-O-Meter by using a 5 inch square piece of the fabric. The fabricwas tested on the Handle-O-Meter in both the Machine and Cross machinedirections.

[0035] The patents and documents referred to herein are herebyincorporated by reference.

[0036] Having described presently preferred embodiments of the presentinvention, it is to be understood that it may be otherwise embodiedwithin the scope of the appended claims.

What is claimed:
 1. A latex which is particularly beneficial forutilization as a binder for nonwoven applications made by the processwhich comprises the steps of: (1) polymerizing a monomer mixturecomprising styrene, itaconic acid, surfactant and water soluble freeradical initiator to form a seed; (2) adding a monomer mixture ofstyrene, butadiene and chain transfer agent and reacting the mixture tobetween 40-95% conversion; (3) sequentially adding equal increments of amonomer mixture of styrene, butadiene, acrylic acid and optionalfunctional monomers, in the presence of a chain transfer agent to (2)under emulsion polymerization conditions and reacting to more than 99%conversion; and then (4) neutralizing the copolymer from (3) to a pH ofabout 4.5 to 7 to form the latex.
 2. The latex of claim 1 wherein themonomer mixture includes about 20-80 wt % styrene.
 3. The latex of claim1 wherein the monomer mixture also includes about 0.5-2.5 wt % itaconicacid, about 20-80 wt % butadiene and about 6-10 wt % acrylic acid. 4.The latex of claim 3 wherein the monomer mixture includes 8 wt %.acrylic acid.
 5. The latex of claim 3 wherein the monomer mixtureincludes 2 wt % itaconic acid.
 6. The latex of claim 1 wherein themonomer mixture includes about 0.05-2.0 wt % surfactant.
 7. The latex ofclaim 1 wherein the copolymer is neutralized by addition of a baseselected from potassium hydroxide, sodium bicarbonate, and ammoniumhydroxide, sodium hydroxide.
 8. A method for making a composite usefulas a personal hygiene article which comprises: (1) treating a nonwovenfabric with a latex binder to form a permeable sublayer, the latexprepared by a process which comprises the steps of: (a) polymerizing amonomer mixture comprising styrene, itaconic acid, surfactant and watersoluble free radical initiator to form a seed; (b) adding a monomermixture of styrene, butadiene and chain transfer agent and reacting themixture to between 40-95% conversion; (c) sequentially adding equalincrements of a monomer mixture of styrene, butadiene, acrylic acid andoptional functional monomers, in the presence of a chain transfer agentto (b) under emulsion polymerization conditions and reacting to morethan 99% conversion; and then (d) neutralizing the copolymer from (c) toa pH of about 4.5 to 7 to form the latex; (2) providing at least onefluid permeable top sheet layer and at least one substantially fluidimpermeable backsheet layer; and (3) interposing the sublayer materialbetween the topsheet layer and backsheet layer.
 9. The method of claim 8wherein the permeable sublayer of the personal hygiene article is a webcomprised of randomly arranged nonwoven fibers having an open structureand high loft.
 10. The method of claim 9 wherein the web is formed bycarding.
 11. The method of claim 9 wherein the fibers are selected fromnatural textile fibers, artificial organic textile fibers, vinyl resinfibers and mixtures thereof.
 12. The method of claim 9 wherein thefibers are selected from polyolefins, polyesters, acrylics, andpolyamides.
 13. The method of claim 12 wherein the polyolefin fibersinclude polypropylene, polyethylene, polybutene and their copolymers.14. The method of claim 12 wherein the polyester fibers include any longsynthetic polymer composed of at least 85% by weight of an ester of adihydric alcohol and terephthalic acid.
 15. The method of claim 12wherein the acrylic fibers include any fiber forming substancecontaining a long chain synthetic polymer composed of at least 85% byweight acrylonitrile units —CH₂ CH(CN)—.
 16. The method of claim 9wherein the fibers are graphite fibers.
 17. The method of claim 9wherein the fibers are of a length of about 0.25 to 2 inches and about1.2-15 denier.
 18. The method of claim 8 wherein the latex binder isapplied on the fiber mat is about 15-40 wt %.